Counterfeit phones can have a fake IMEI number. Carriers typically block phones with IMEI numbers that have been reported stolen, and some carriers also block phones with faked IMEI numbers.
IMEI numbers are 15 digits long, and databases, which owners can access online, carry information involving the phone's manufacturer, home country and model type. Owners who think they might have a phone with a fake IMEI can dial *#06# to get the phone's IMEI number; if this number doesn't match the one listed on the phone itself, it's likely faked. Online tools can help users investigate a suspicious number as well. If the tool returns a mismatched phone model, for example, the phone's IMEI number is likely fake.